Friction stir welding member

ABSTRACT

This invention provides a configuration of a joint that allows a satisfactory welded joint to be formed with reduced deformation of the joint region when two-face structures (panels) are friction-welded end to end. The panels  31, 32  each have two substantially parallel plates  33, 34  and a third member  35  connecting the two plates  33, 34 . The end portions of the plates  33, 34  of one panel  32  are friction-welded to the end portions of the plates  33, 34  of the other panel  32 . At least one of the panels has a plate  36  at its end for connecting the plates  33  and  34  and has a rigidity to support a pressing force produced during the friction welding.

[0001] This application is a Divisional application of application Ser.No. 08/820,231, filed Mar. 18, 1997, the contents of which areincorporated herein by reference in their entirety.

BACKGROUND OF THE INVENTION

[0002] The present invention relates to a friction stir welding methodthat is applicable to panel welding used, for example, in aluminum alloyrailway cars and buildings.

[0003] A two-face structure (panel) for railway cars using hollowmembers is disclosed in Japanese Patent Laid-Open No. 246863/1990, andanother using laminated panels, such as honeycomb panels, is disclosedin Japanese Patent Laid-Open No.106661/1994.

[0004] The process of friction stir welding is performed by rotating around rod inserted in a joint region to heat and plasticize the jointregion thus forming a weld. This type of welding is generally applied toa butt joint and a lap joint, and is described in WO 93/10935 (which isthe same as EP 0615480B1 and the Japanese Announcement laid-openpublication No. Hei 7-505090 and in the publication Welding & MetalFabrication, January 1995, pp. 13-16.

SUMMARY OF THE INVENTION

[0005] In friction stir welding, the reaction of the plasticized metalbeing extruded from immediately beneath the rotating tool (round rod) tothe surface during the welding resplts in a downward force acting on thejoint region. Thus, when this welding method is applied to a two-facestructure (panel), this downward force causes the joint material at thejoint region to flow downward, deforming the joint. This makes itimpossible to produce a satisfactory weld.

[0006] Two-face structures (panels) include hollow members made ofextruded aluminum alloy and honeycomb panels. Joining such panels hasbeen accomplished by MIG welding and TIG welding. When friction stirwelding is applied to such a joint, the joint is bent down or thematerial in the joint region is forced to flow down due to a downwardforce produced during the friction stir welding.

[0007] The inventor has found the above phenomena in a variety ofexperiments.

[0008] It is a first object of this invention to provide a satisfactorywelded joint by minimizing deformation of the joint region when twofaces are friction stir welded.

[0009] It is a second object of this invention to provide a satisfactorywelded joint when one face is friction stir welded.

[0010] It is a third object of this invention to enable two faces to bewelded together in a short time with little deformation.

[0011] The first object can be achieved by placing at the joint region aconnecting member that joins two plates forming two faces.

[0012] The second object is realized by providing the members at thejoint region with a raised portion that protrudes toward the frictionstir welding tool side.

[0013] The third object is realized by disposing rotary tools forfriction stir welding on both sides of the objects to be welded, placingthe rotation center of one of the tools on an extension of the rotationcenter of the other tool, and performing friction stir weldingsimultaneously.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014]FIG. 1 is a vertical cross section of one embodiment of thisinvention.

[0015]FIG. 2 is a vertical cross section of FIG. 1 after friction stirwelding.

[0016]FIG. 3 is a vertical cross section of another embodiment of thisinvention.

[0017]FIG. 4 is a vertical cross section of FIG. 3 after friction stirwelding.

[0018]FIG. 5 is a vertical cross section of another embodiment of thisinvention.

[0019]FIG. 6 is a vertical cross section of FIG. 5 after friction stirwelding.

[0020]FIG. 7 is a vertical cross section of still another embodiment ofthis invention.

[0021]FIG. 8 is a vertical cross section of FIG. 7 after friction stirwelding.

[0022] FIGS. 9(A) to 9(D) are vertical cross sections showing steps inthe procedure of friction stir welding of a further embodiment of thisinvention.

[0023]FIG. 10 is a vertical cross section of a further embodiment ofthis invention.

[0024]FIG. 11 is a vertical cross section of a further embodiment ofthis invention.

[0025]FIG. 12 is a vertical cross section of a further embodiment ofthis invention.

[0026]FIG. 13 is a vertical cross section of a further embodiment ofthis invention.

[0027]FIG. 14 is a vertical cross section of a further embodiment ofthis invention.

[0028]FIG. 15 is a perspective view of a structural body of a railwaycar.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0029] The embodiment shown in FIG. 1 has a joint configuration of theabutting type between hollow members 31, 32 which are in the form ofpanels. The hollow members 31, 32 have vertical plates 36, 36 at theirends in the width direction. Before the welding, the vertical plates 36,36 are disposed immediately beneath a rotary tool 50. The verticalplates 36, 36 are opposed to and in contact with each other. If they arespaced apart, the distance is small and approximately 1 mm. On theextension of the interface between the vertical plates 36, 36 lies thecenter of a projection 52. The vertical plates 36, 36 have a stiffnesswhich is sufficiently strong to sustain the downward force mentionedearlier. The vertical plates 36 are perpendicular to two plates 33, 34in each panel. The hollow members 31, 32 are formed by extruding analuminum alloy. The upper and lower faces of the hollow member 31 areflush with the corresponding upper and lower faces of the hollow member32. That is, the hollow members 31, 32 have the same thickness. This istrue also of the succeeding embodiments. During the friction stirwelding process, the boundary 53 between a large-diameter portion 51 andthe projection 52 of a small-diameter of the rotary tool 50 is situatedabove the upper surfaces of the hollow members 31, 32. Numeral 35designates a plurality of members that are arranged in the form oftrusses to connect the two plates 36, 36. The hollow members 31, 32 eachhave bilaterally symmetrical end portions and are mounted on a bed (notshown) where they are fixed immovably. The bed also lies under thevertical plates 36, 36.

[0030] The friction stir welding process is performed by rotating thetool 50, plunging the projection 52 into the joint region of the hollowmembers 31, 32, and moving the projection 52 along the joint region. Therotating center of the projection 52 is located between the two verticalplates 36, 36.

[0031]FIG. 2 shows the two panels after they have been friction stirwelded. Reference number 45 denotes the shape of a weld bead afterwelding. On the extension of the border line between the vertical plates36, 36 the width center of the weld bead 45 is situated. The bead 45lies in an area on the extension of the thickness of the vertical plates36, 36. The depth of the weld bead 45 is determined by the height of theprojection 52 at the lower end of the rotary tool 50 inserted in thejoint region.

[0032] With this construction, because the vertical plates 36, 36perpendicular to the plates 33, 34 sustain the vertical force producedduring the friction stir welding, the joint region does not bend,offering a satisfactory joint as shown in FIG. 2. The vertical plate 36is made perpendicular to the plates 33, 34 in each panel as much aspossible.

[0033] The vertical plate 36 may be perforated to achieve a lighterweight. This is true also of the succeeding embodiments.

[0034] Welding of the lower side is carried out by turning the hollowpanel members upside down after the welding of the one side iscompleted.

[0035] The embodiment of FIG. 3 has a vertical plate 36 at the end ofone hollow member 31, but not at the opposing end of the other hollowmember 32. Corners in the vertical direction of the vertical plate 36 ofthe hollow member 31 are recessed so as to receive the ends ofprojecting pieces 38, 38 of the hollow member 32. These recessedportions are open in a direction of the thickness of the hollow member31 and in a direction perpendicular to the thickness direction (towardthe hollow member 32 side). When the projecting pieces 38 are placed(superposed) on the recessed portions, there is actually a clearancebetween them although they are in contact with each other in the figure.There is also a gap between the front ends of these members (i.e.,between the projecting pieces 38, 38 and the corners 33 a, 34 b). Theabutting joint portions on the upper face side of the two hollow members31, 32 and the vertical plate 36 are situated directly below the centerof the rotary tool 50. The rotating center of the projection 52 of thewelding tool 50 is disposed on an extension of the center line of thethickness of the vertical plate 36. That is, the joint region of theplate 33 (34) and plate 33 (34) is situated on the extension of thecenter line of the thickness of the vertical plate 36. The corners 33 b,34 b extending from the plates 33, 34 to the recessed portions lie on anextension of the center line of the thickness of the vertical plate 36.Considering the gap between the corners 33 b, 34 b and the projectingpieces 38, the corners 33 b, 34 b are situated slightly to the left ofthe extension of the center line of the thickness of the vertical plate36. The vertical plate 36 has a rigidity to support the downward force.The horizontal gap between the front ends of the projecting pieces 38and the hollow member 31 is similar to that shown in FIG. 1. The heightof the projection 52 of the welding tool 50 is approximately equal tothe thickness of the projecting piece 38. The region that is plastic andfluid extends below the projecting piece 38, and comes to have an arealarger than the diameter of the projection 52, and the two hollowmembers 31, 32 are friction stir welded. It is desirable that thefriction stir weld be so formed as to extend beyond the contact areabetween the underside of the projecting piece 38 and the vertical plate36.

[0036]FIG. 4 shows the state of the joint after being welded. The weldbead 45 is formed such that the width center of the weld bead 45 issituated on an extension of the thickness center of the vertical plate36.

[0037] To support the vertical force, it is desirable for the rotatingcenter of the tool 50 to be located on the extension of the center lineof the thickness of the vertical plate 36. To make the quantity of thejoint of the left and right hollow members 31, 32 equal, it is desirablefor the corners 33 b, 34 b to be situated on the extension of thethickness center line of the vertical plate 36. While the projection 52of the tool 50 should preferably be placed in the range betweenextension lines of the thickness of the vertical plate 36, the thicknessof the vertical plate 36 is determined by the vertical force, theposition of the projection 52 and the strength of the vertical plate 36.Hence, there may be a case in which the thickness of the plate 36 issmaller than the diameter of the projection 52. In view of the possibleerrors of the position of the rotary tool 50 and those of the corners 33b, 34 b, it is desirable for the corners 33 b, 34 b to be positioned inthe range between extension lines of the thickness of the vertical plate36, and at least a part of the projection 52 of the tool 50 to besituated in this range. This arrangement enables the vertical plate 36to receive at least a part of the vertical force, substantiallypreventing the deformation of the joint. As a result, a satisfactoryjoint can be formed. When the bead 45 is taken as a reference, althoughthe bead 45 is slightly larger than the projection 52, the same as abovecan be said. This is true also of the other embodiments.

[0038] Compared with the case of FIG. 1, this joint configuration canminimize a sinking in the surface of the joint region even when thehorizontal gap between the projecting piece 38 and the hollow member 31is large. As a result, the joint has a good appearance and requires areduced amount of putty for painting. This is because the gap betweenthe two members is terminated at a depth equal to the thickness of theprojecting piece 38. It is also considered that this joint configurationcan reduce the weight. Further, because one of the hollow members isfitted into the other, the positioning in the height direction of thetwo members can be accomplished easily.

[0039] The ends of the hollow member 31 are bilaterally symmetrical inshape. The ends of hollow member 32 are also bilaterally symmetrical.Alternatively, the hollow member 31 may have one end shaped as shown inFIG. 3 and the other end shaped like the end of the hollow member 32 ofFIG. 3.

[0040] In the embodiment of FIG. 5, there is virtually no vertical plate36 immediately below the corners 33 b, 34 b of the recessed portion ofthe hollow member 31. The right end of the vertical plate 36 lies on theextension of the corners 33 b, 34 b. On this extension the rotatingcenter of the tool 50 is located. The end portion of the hollow member31 is given a rigidity to sustain the vertical force by making the lowerprojecting piece 37 at the joint thicker and increasing the size of thearcs extending from the front ends of the projecting pieces 37 to theplate 36. The projecting pieces 38 of the other hollow member 32 arereceived in the recessed portions of the projecting pieces 37, as in thepreceding embodiment of FIG. 3. The second hollow member 32 has avertical plate 36 near the projections 38 for connecting the upper andlower plates 33, 34. This arrangement prevents the joint region frombeing defective even when there is no vertical plate 36 directly belowthe corners of the recessed portions. It is noted, however, that belowthe range of the bead 45 there is a vertical plate 36 of the panel 31.FIG. 6 shows the state after welding.

[0041] In the embodiment of FIG. 5, the plate 36 of the hollow member 32may be removed.

[0042]FIG. 7 shows another embodiment, representing a variation of thepreceding embodiment of FIG. 5, in which the joint region of the twohollow members 31, 32 is provided with raised portions 37 a, 38 aprotruding outside. This makes the joint region thick. The heights ofthe raised portions 37 a, 38 a are equal. Other parts are similar tothose of FIG. 5, except that the vertical plate 36 and the projections37 are slightly thinner.

[0043] With this configuration, if there is a gap between the raisedportions 37 a and 38 a before welding, the gap is filled with thematerial of the raised portions 37 a, 38 a, when welded, improving theappearance and reducing the amount of putty required.

[0044] In conventional welded joints, the weld bead has a sunken portionor recess corresponding to the volume of the lost material 41 that hasflowed down due to the downward force. In the joint configuration ofFIG. 7, the rotary tool 50 plasticizes the raised portions 37 a, 38 aand forces them downward making up for the lost volume of the material41. Thus, formation of recess can be prevented, providing a satisfactorywelded joint. FIG. 8 shows the shape of bead 45 after welding. Afterwelding, unnecessary parts, if any, are cut off as shown.

[0045] The raised portions 37 a, 38 a can also be applied to theembodiments of FIGS. 1, 3 and 5 and to subsequent embodiments.

[0046] FIGS. 9(A)-9(D) show a further embodiment, which allows weldingat the upper and lower faces from only one side. The ends of the hollowmembers 31, 32 on the lower side having projecting pieces 34 a, 34 aprotruding flush with the lower plates 34, 34 substantially toward theopposing hollow member sides. The front ends of the projecting pieces 34a, 34 a are virtually in contact with each other. The front ends of theupper plates 33, 33 are offset back from the front ends of the lowerplates 34 a, 34 a. The front ends of the upper plates 33, 33 areconnected to the lower plates 34, 34 through the vertical plates 36, 36.The vertical plates 36, 36 are connected to intermediate portions of thelower plates 34. The top portions of the vertical plates 36, 36 areprovided with recessed portions 39, 39 that receive a joint 60. Whenmounted on the recessed portions 39, 39, the upper surface of the joint60 is flush with the upper face of the upper plates 33, 33. The distancebetween the two vertical plates 36, 36 is long enough for the rotarytool 50 to be inserted and is as short as possible. The relation betweenthe vertical plates 36 and the recessed portions 39 is the same asexplained with reference to the embodiments of FIG. 5 and FIG. 7.

[0047] The welding procedure will be described below. In the state ofFIG. 9(A), the abutting ends of the lower plates 34 a, 34 a are weldedby the rotary tool 50. At this time, the hollow members 31, 32,including the joint region of the plates 34 a, 34 a, are mounted on abed. The upper surface of the bed (that backs the bead) is flat. Theheight of the projection 52 of the rotary tool 50 is smaller than thethickness of the plates 34 a, 34 a. This design ensures that the bottomsurface after welding is flat. Thus, the bottom side can easily be usedas an outer surface of the structure of a railway car or a building (theouter surface being the surface on which no decorative plate ismounted). Generally, the upper face of the friction stir welded jointtends to be uneven (at a boundary portion 51).

[0048] Next, as shown in FIG. 9(B), the joint 60 is mounted between thetwo hollow shape members 31, 32. The joint 60 in the illustrated exampleis T-shaped in vertical cross section. When both ends of the joint 60are placed on the recessed portions 39, 39, the lower end of a verticalportion 61 has a clearance between it and the weld bead on the lowerplate. The vertical portion 61 may be omitted.

[0049] Next, as shown in FIG. 9(C), the joint portion between the joint60 and the hollow member 31 is friction stir welded by the rotary tool50. The rotary tool 50 need not be the same as used for the weldillustrated in FIG. 9(A).

[0050] Then, as shown in FIG. 9(D), the joint portion between the joint60 and the hollow member 32 is friction stir welded by the rotary tool50.

[0051] This procedure allows the welding to be performed from one sideand eliminates the need for inversion of the panels. With the inversionof the panels eliminated, there is an advantage that the time requiredfor inversion and positioning and need for an inversion device areunnecessary, and even the assembly precision is improved.

[0052]FIG. 10 shows another embodiment, in which both the upper andlower sides of the hollow members 51, 52 are friction stir welded at thesame time. A rotary tool 50 a for the lower side is disposed verticallybelow the welding tool 50 for the upper side. The projection 52 of thesecond welding tool 50 a faces up. The two welding tools 50, 50 a facingeach other are moved at the same speed to perform friction stir welding.Denoted by 70, 70 are beds (tables). The rotating centers of the tools50 and 50 a are on the same line on which the joint region of the hollowshape members 31, 32 is located.

[0053] Because with this arrangement the rotating center of the secondtool 50 a is positioned on the extension of the rotating center of thefirst tool 50, the forces applied to the panels balance with each otherallowing the joint to be welded in a short time with little deformation.Further, because there is no need to invert the hollow members 31, 32,the welding can be performed in a short time with little deformation ofthe joint.

[0054] This procedure can be applied to other embodiments.

[0055] The preceding embodiments have used hollow members as panels tobe joined. The following embodiments show friction stir welding asapplied to honeycomb panels. As shown in FIG. 11, the honeycomb panels80 a, 80 b comprise two surface plates 81, 82, core members 83 havinghoneycomb-like cells, and edge members 84 arranged along the edges ofthe surface plates 81, 82, with the core members 83 and the edge members84 soldered to the surface plates 81, 82 to form integral structures.The surface plates 81, 82, the core members 83 and the edge members 84are made of aluminum alloy. The edge members 84 are made by extrusionand have a rectangular cross section. All sides of this rectangularcross section are greater in thickness than the surface plates 81, 82.The vertical sides of the mutually contacting edge members 84, 84 havethe same thickness as shown in FIG. 1. The two honeycomb panels 80 a, 80b have the same thickness.

[0056] The welding procedure in the embodiment of FIG. 11 corresponds tothe one shown in FIG. 1. The height of the projection 52 of the rotarytool 50 is larger than the thickness of the face plates 81, 82. Thisallows the face plates 81, 82 and the edge members 84, 84 to be welded.The load acting on the panels 80 a, 80 b is transmitted mainly by theedge members 84. After being fabricated, the panels 80 a, 80 b areassembled and friction stir welded.

[0057] The embodiment of FIG. 12 corresponds to the one shown in FIG. 3.The edge member 84 of the honeycomb panel 80 a has a generallyrectangular cross section and has recesses at the corners. The edgemember 84 of the honeycomb panel 80 b is like a channel, with itsopening facing the honeycomb panel 80 a. The open ends of the edgemember 84 are mounted on the recessed portions of the edge member 84 ofthe honeycomb panel 80 a.

[0058] The honeycomb panel corresponding to FIG. 5 can be fabricated ina similar manner.

[0059]FIG. 13 shows still another embodiment that corresponds to FIG. 7.After two honeycomb panels 80 a, 80 b are assembled, a plate 86 isplaced on the face plates 81, 81 and temporarily welded to them. Theplate 86 makes up for the material that is plasticized and flows out. InFIG. 12, one vertical piece of the edge member of the honeycomb panel 80a is removed. The vertical force is supported by the thickness of thehorizontal piece of the edge member 84 and the surrounding parts.

[0060]FIG. 14 shows a further embodiment of this invention. Thepreceding embodiments up to FIG. 13 include panels having two faces(face plates), whereas the embodiment of FIG. 14 includes panels 91, 92having virtually a single face (face plates 94, 94). Friction stirwelding is performed at two locations, at the abutting ends of thepanels 91, 92, the outside with face plates 94 and the inner side withno face plates. Therefore, the joint regions on the inner side areprovided with narrow face plates (face plates 93, 93). The narrow faceplates 93, 93 are supported by vertical plates 96, 96. In this example,too, the vertical plates 96 are virtually perpendicular to the faceplates 93, 94. The face plates 93, 94 are provided with raised portions37 a, 38 a similar to the ones shown in FIG. 7. The face plates 94, 94have a plurality of reinforcing ribs (plates) 95, 95 at specifiedintervals. The ribs 95 are T-shaped in cross section. The top surfacesof the ribs 95 are flush with those of the face plates 93 of the jointregion. To the top surfaces reinforcing members (such as pillars) may bewelded, or the top surfaces serve as mounting seats for articles.Further, the face plates 93, 93 also serve as a seat for controlling theheight of the tool 50. A movable body carrying the tool 50 travels alongthe face plates 93, 93. Because of the provision of the face plates 93,94, the panels 91, 92 can also be said to form a two-face structure. Thepanels 91, 92 are extruded shape members.

[0061] While FIG. 14 shows the vertical plates 96, 96 of the panels 91and 92 opposing each other at the joint region, as in the configurationof FIG. 1, it is possible to place one of the vertical plates over theother, as shown in FIGS. 3, 5 and 7.

[0062]FIG. 15 shows an example of application of this invention to thestructural body of a railway car. The structural body has side bodies101, a roof body 102, a floor body 103, and gable bodies 104 at the endsin the longitudinal direction. The side bodies 101 and the roof body 102have panels 31, 32, 80 a, 80 b, 91, 92 whose long sides are oriented inthe longitudinal direction of the car. The joint between the side bodies101 and the roof body 102 and between the side bodies 101 and the floorbody 103 is accomplished by MIG welding. The roof body 102 and the sidebodies 101 are often shaped into arcs in cross section. When the panels91, 92 are used for the side bodies 101, the side having the verticalplates 96 and ribs 96 is made to face to the interior of the car and thereinforcing members constitute pillars.

[0063] The panels 31, 32 of FIG. 9 may be combined in a mirror-imagearrangement. The end of the projecting plate 34 a of each panel isplaced on the recessed portion 39 of the plate 33 of the other panel.This obviates the use of the joint 60 and allows for simultaneousfriction stir welding of the joint region both from above and below. Theplates 33, 34 a can be provided with raised portions, as shown in FIG.7.

What is claimed is:
 1. A member adapted to be used in friction stirwelding, comprising: a member having a recessed portion along an endportion of said member, said recessed portion opening directed toward anouter side in a thickness direction of said member and toward one enddirection of said member, a raised portion projecting to an outer sidein the thickness direction, from an outer side of said member, and saidraised portion is a portion adapted to have a friction stir weldingcarried out therein by inserting a rotary tool therein.
 2. A hollowframe member adapted to be used in friction stir welding, comprising: afirst plate, a second plate which is substantially in parallel to saidfirst plate, and a third plate connecting said first plate and saidsecond plate, said first plate having a recessed portion in a side of anouter side of said first plate along to an end portion of said firstplate, said recessed portion opening directed toward an outer side in athickness direction of said hollow frame member and toward one enddirection of said hollow frame member, a raised portion in a side ofanother end of said first plate, connected to said recessed portion andprojecting to an outer side in the thickness direction from an outerside of said first plate, and said raised portion is a portion adaptedto have a friction stir welding carried out therein by inserting arotary tool therein.
 3. A hollow frame member according to claim 2,wherein said recessed portion is provided at a connection portion ofsaid third plate and said one end portion of said first plate.
 4. Ahollow frame member according to claim 3, wherein: said third plate ofsaid first member is substantially orthogonal to said first plate, and aface from an apex of said raised portion to a bottom face of saidrecessed portion is positioned in a range of an extension line in athickness of said third plate.
 5. A hollow frame member according toclaim 4, wherein said face from said apex of said raised portion to saidbottom face of said recessed portion is positioned at an extension lineof a center in a thickness of said third plate.
 6. A hollow frame memberaccording to claim 4, wherein said face from said apex of said raisedportion to said bottom face of said recessed portion is positioned atanother end side of said first plate from a center in a thickness ofsaid third plate.
 7. A hollow frame member according to claim 2,wherein: at a side of said end portion of said first plate, having saidrecessed portion, a second recessed portion is provided along to saidone end of said second plate, said second recessed portion opensdirected toward an outer side in a thickness direction of said hollowframe member and toward said one end direction of said first plate, at aside of another end of said second plate beyond said second recessedportion, a second raised portion connecting to said second recessedportion and projecting to an outer side in the thickness direction froman outer side of said second plate is provided, and said second raisedportion is a portion adapted to have a friction stir welding carried outtherein by inserting a rotary tool therein.
 8. A hollow frame memberaccording to claim 7, wherein said second recessed portion is providedat a connection portion of said third plate and said one end of saidsecond plate.
 9. A hollow frame member according to claim 7, wherein:said third plate is substantially orthogonal to said second plate, and asecond face from an apex of said second raised portion to a bottom faceof said second recessed portion is positioned in a range of an extensionline in a thickness of said third plate.
 10. A hollow frame memberaccording to claim 8, wherein a second corner portion, of said secondrecessed portion, is positioned at an extension line of a center in athickness of said third plate.
 11. A hollow frame member according toclaim 8, wherein a second corner portion is positioned at another endside of said second plate from a center in a thickness of said thirdplate.